Filter device

ABSTRACT

The filter device for filtering liquids comprises a filter membrane ( 2 ), a main body which is arranged upstream from the filter membrane ( 2 ),and which has a feed channel ( 3 ) perpendicular to the surface of the filter membrane ( 3 ), a gap ( 5 ) extending between the filter membrane ( 2 ) and the main body forming a tangentially directed cross-flow, and a return channel ( 6 ) which is used to return the tangential flow to the feed channel. Said flow is primarily returned thereto by means of a contraction ( 3 ′) in the feed channel ( 3 ) at the mouth of the return channel ( 6 ). Preferably, a solid separator ( 20 ) is disposed in the return channel. At least one chicane element ( 7, 8, 9 ) is arranged in the feed channel in order to create a turbulent flow. The tangential flow along the filter membrane is used to clean said membrane.

[0001] The invention pertains to a filter device for filtering fluidsaccording to the preamble of Claim 1.

[0002] A filter device of this type is known from DE 72 25 662 U1. Thisfilter device is used for tangential or crossflow filtration processes,in which a predominantly tangential flow is produced along the membrane.

[0003] Another filter device for filtration of fluids containing solidsby means of at least one filter surface is known from DE 39 24 658 A1.In order to prevent the formation of filter cakes and the clogging ofthe filter pores, this publication proposes to set the filter surface inmotion. Alternatively or additionally, a flow can be produced in thefluid to be filtered, wherein the velocity component of said flow thatis directed tangentially relative to the filter surface is no lower than0.1 m/sec. A turbulent flow is preferably produced in this case in orderto keep the filter surface clean, for example, by means of a passingflow consisting of a gas/fluid mixture. However, the desired turbulencefor counteracting the formation of filter cakes can also be producedwith pumps, nozzles, agitating units, baffles, guide plates, rotatingrollers or vibration devices. In addition, it would also be conceivableto electrically charge the filter surface such that it repels solids.

[0004] The known device comprises several dirty water chambers and acorresponding number of pure water chambers that are separated from oneanother by a membrane. The main flow direction in the dirty waterchambers extends tangentially to the membrane. The fluid to be filteredis also supplied parallel to the membrane. Another important aspect isthat the filtration takes place in a largely unpressurized fashion. Anenrichment of the solids contained in the fluid occurs in front of thefilter surfaces in the dirty water chambers, and the solids are thenremoved from the filter surfaces by means of lines and pumps. Dependingon the turbidity of the fluid, this should take place in the dirty waterchambers. A measuring head and a control unit for controlling thecorresponding pump are also provided in this case.

[0005] Blocking or “fouling” of the filter membranes occurs in membranefiltration methods,. This causes the rate of flow through the filtermembrane or the filter medium per unit time (dV/dt) to decrease. Varioussub-processes contribute individually or collectively to this fouling.First, the pores of the membrane are partially blocked such that arelatively rapid decrease in the rate of flow results; second, aso-called “filter cake” (frequently also relative to as “secondarymembrane”) is formed on the upstream side of the filter medium. Thisfilter cake additionally increases the flow resistance such that therate of flow through the filter medium is additionally reduced, but itincreases the resistance more slowly than does blocking of the pores. Aso-called concentration polarization layer may also form on the filtercake.

[0006] The various processes that cause fouling can be categorized intoreversible and irreversible processes that can either be reversed withcertain measures, e.g., backflushing, or not reversed (e.g.,adsorption).

[0007] In addition, the retained materials not only form a filter cakeon the upstream side of the filter (inflow side), but penetrate at leastpartially into the filter medium and block the channel or porestructures therein. In such instances, cleaning of the filter medium bymeans of conventional cleaning methods can, if at all, be achieved onlywith great difficulty.

[0008] These mechanisms reduce the maximum operating time (service life)of the filter media and consequently increase the costs and expenditureof labor (maintenance, monitoring, etc.) for the filtration process.

[0009] Numerous proposals, methods, and devices for removing a filtercake are known from the prior art. Physical and chemical methods areutilized for removing the retained materials and for cleaning thefilter.

[0010] The physical cleaning methods include, for example, the use ofraking or shaking devices. In methods of this type, the filter cake isremoved from the upstream side of the filter medium by a rake or asimilar device. Sometimes the filter is set into vibration or aturbulent flow is produced.

[0011] For example, baffles and/or frequency generators (such asultrasound generators) are used to produce turbulent flows. In a fewembodiments known from the prior art, a flow that is directed parallelto the filter surface (crossflow) is produced in order to carry offfilter cake particles, with the aid of a suitably introduced gas, forexample. However, such mechanisms require large filter surfaces forachieving a sufficient flow through the filter medium.

[0012] So-called “backwashing” of the filter also falls into thiscategory of cleaning methods. In this method, the filtration process isinterrupted, the filter medium is flushed with a cleaning fluid in thedirection opposite to the original flow, and the filtration process issubsequently continued (with a certain “lead time”).

[0013] In chemical methods, the filter medium is, for example, flushedwith one or more cleaning solutions. These cleaning solutions may remainin the filter medium after the cleaning process and, if so required,need to be flushed out (for example, by continuing the filtrationprocess and discarding rather than using fluid filtered immediatelyafter the filtration process is restarted).

[0014] Known methods for removing a filter cake (e.g., DE 39 24 658 A1)have certain disadvantages. They require a substantial technicalexpenditure (raking or agitating units, gas supply, vibration devices,etc.), and the methods become more complicated and susceptible todefects. In addition, the maintenance effort and costs are comparativelyhigh.

[0015] Conventional cleaning methods are not always entirely effective,i.e., they do not completely clean the filter membrane or its inflowside.

[0016] Another disadvantage can be seen in the fact that certain devicesknown from the prior art require large systems, filter surfaces, etc.,in order to achieve a sufficient filtrate throughput.

[0017] Another disadvantage of these cleaning methods is that filtrationneeds to be interrupted in order to carry out the cleaning process. Thisreduces filter throughput and increases costs.

[0018] Another disadvantage can be seen in the energy requirement ofthese conventional methods and the associated costs.

[0019] According to DE 38 18 437 A1, which also forms part of thepertinent prior art, a filter battery with baffle elements is used inorder to produce turbulences. The battery is alternately operated in thefiltration mode and in the flushing mode.

[0020] DE 43 29 587 C1 discloses a self-cleaning device for filtering afluid, wherein the filter is integrated into the wall of a pipe and thefluid to be filtered is supplied to the pipe. The fluid to be filteredpartially passes through the filter and emerges in the form of filtrate,and the remainder of the fluid to be filtered, which still containssolids, emerges from the outwardly opening end of the pipe.

[0021] WO 97/32652 discloses a filter in which the medium to be filteredis also subjected to intense turbulence that is, for example, producedwith a rotor, sound waves, electric fields and the like.

[0022] DE 35 20 489 C1 discloses a filter with a tangential inlet,wherein the fluid is introduced into the interior of a receptacle with acontrolled turbulence and an agitator arranged above the filter membraneproduces a continuously changing field of alternating pressure in orderto prevent the membrane from becoming covered with particles.

[0023] DE 14 36 287 A1 also discloses a tangential filter, in which thefilter wall is flushed in order to loosen deposits.

[0024] The invention is based on the objective of improving known filterdevices in such a way that the formation of a cover layer (filter cake)of materials retained by the filter membrane is, if not entirelyprevented, at least significantly slowed on the upstream side of thefilter membrane (inflow side).

[0025] This should essentially be achieved by automatically cleaning theinflow side of the filter membrane. It should, in particular, bepossible to clean the filter membrane in a largely continuous fashionwithout external interventions by the personnel, with the fewestpossible interruptions of the filtration process, and without auxiliarymedia (e.g., introduced gases). Replacement of the filter arrangement orof one of its components preferably should also be simplified, and themaintenance effort should be reduced. In addition, the service life ofthe filter membrane should be extended while simultaneously reducingcosts.

[0026] This objective is attained with the characteristics disclosed inClaim 1. Advantageous embodiments and additional developments of theinvention are disclosed in the dependent claims.

[0027] The basic principle of the invention consists of producing acirculation, i.e., a crossflow for cleaning the filter membrane and areturn flow, wherein the main filtering direction lies perpendicular tothe membrane. This makes it possible to continuously remove and disposeof the retained materials without having to interrupt the filtrationprocess. In contrast to the initially cited DE 72 25 662 U1, theinvention proposes to predominantly carry out a so-called “dead-endfiltration,” wherein the main flow is directed perpendicularly throughthe filter membrane and the crossflow component impressed on the fluidprimarily serves to remove retained materials on the inflow side of thefilter membrane. The fluid of the crossflow component that transportsthe retained materials is conveyed to a return channel via extractionpoints and conveyed back into the supply channel via this returnchannel. A solids separator for additionally depleting the solidscontent of the fluid preferably is arranged in the region of the returnchannel.

[0028] The driving force for the filtration process is generated by adevice that produces the main flow in the supply channel, e.g., a mainpump.

[0029] The tangentially directed crossflow is achieved due to thegeometry of the filter membrane and the main body assigned thereto. Thereturn flow in the return channel, which has a substantial component inthe opposite direction of the main flow in the supply channel, ispreferably realized by means of a flow restriction that causes localpressure differentials at the point at which the return channel feedsthe fluid back into the supply channel. Due to the relatively shortdistances and the low pressure drop along these distances, only arelatively slight pressure differential is required for the return flowof the fluid. The correlation between pressure and flow velocity andbetween channel cross section and flow velocity is described inaccordance with Bernoulli's equation$P_{gas} = {{p + {\frac{1}{2}\rho \quad v^{2}}} = {{konst}.}}$

[0030] (with p=pressure; ρ=density and v=velocity) and with thecontinuity equation

A ₁ V ₁ =A ₂ V ₂

[0031] (A=cross-sectional surface perpendicular to the flow velocity).According to these two equations, the velocity increases and thepressure drops as the cross section decreases. If a flow restriction isarranged in the supply channel at the location at which the returnchannel feeds the fluid back into the supply channel, it is possible togenerate a pressure differential between this location and the surfaceof the filter membrane. This pressure differential causes or at leastpromotes the return flow of the fluid.

[0032] Alternatively or additionally, a pump can be arranged in thereturn channel.

[0033] The end of the return channel is preferably realized in the formof a pipe that protrudes into the supply channel and is bent in thedirection of the filter membrane. The cross section of the supplychannel is narrowed in the vicinity of the pipe end, by means of areduced cross section or by “baffle elements” inserted into this region.Such a baffle element consists, for example, of an axially symmetricalinsert that has the function of locally producing a pressuredifferential directly after the pipe end in order to feed the returnedfluid into the supply channel. In this case, the faster flow velocity ofthe fluid caused by the restriction generates a suction effect.

[0034] In conventional tangential or crossflow filtration processes, thethroughput per unit surface area of the filter and unit time is verylow. This means that very large filter surfaces are required in order toachieve a higher, and consequently more economical throughput. Theseparation effect of the filter is predominantly realized in the form ofa diffusion through the filter.

[0035] The device according to the invention is very compact andrequires relatively small filter surfaces. The filter device accordingto the invention also has a high throughput, namely because it is notutilized in a so-called crossflow method, but rather in a so-calleddead-end method (the fluid flows against the membrane parallel to a linenormal to the membrane). Higher pressures are also utilized in thedead-end method, such that a compact design and high throughput of thefilter device can be achieved. The equipment expenditure for the filteraccording to the invention is low because no moving parts are required.This significantly reduces the costs, the susceptibility to defects, theenergy consumption and the expenditure of labor (maintenance,operation).

[0036] The baffle elements used in one preferred embodiment of theinvention serve primarily to produce the required crossflow over theentire membrane surface, such that essentially all membrane regions arereliably cleaned. The turbulence is predominantly produced by acombination of velocity and small gap dimensions, wherein the baffleelements function just to add to the turbulence. This geometry alsomakes it possible to operate with high pressures and consequently a highthroughput. The invention allows very high crossflow velocities that canbe realized without auxiliary means, e.g., pumps, nozzles, agitatorsetc., and consequently without additional energy consumption. These highcrossflow velocities ensure an excellent transport effect, as well asproduction of the required turbulence. The design according to theinvention with replaceable baffle elements can also be easily adapted todifferent process requirements.

[0037] The desired cleaning effects are additionally intensified ifelectric fields are incorporated into the cleaning process.

[0038] The filter arrangement according to the invention not only servesto clean the filter membrane and to dispose of the separated materials,but also protects the membrane from pressure surges to a certain degree.

[0039] The invention is described in greater detail below with referenceto embodiments that are illustrated in the figures. The figures show:

[0040]FIG. 1, a filter device according to the invention in the form oftwo embodiments that are respectively illustrated in the left and in theright half of FIG. 1;

[0041]FIGS. 2a-2 d, different variations of the return flow and of thesolids transport;

[0042]FIGS. 3a and 3 b, a modification of the baffle element and themain body;

[0043]FIG. 4, an embodiment for displaying the installation of baffleelements into the filter device according to the invention, and

[0044]FIG. 5, an alternative embodiment of the installation of a baffleelement into the filter device according to the invention.

[0045]FIG. 1 shows some variations of filter devices according to theinvention. A fluid to be filtered is supplied to a filter membrane 2 ofthe filter device 1 via a supply channel 3 that extends perpendicular tothe surface of the filter membrane 2. The fluid normally consists of afluid that is contaminated with solids. However, it may also consist ofa gas that is contaminated with solids. A main body 4 is arranged infront of the filter membrane 2 viewed in the flow direction (see arrowF_(in)). This main body continues and consequently forms part of thesupply channel 3, and has a contour that extends laterally beyond thesupply channel 3 essentially parallel to the filter membrane 2. Thiscontour lies a short distance B from the filter membrane and forms a gap5. In the left portion of FIG. 1, this gap has a constant with B. In thevariation illustrated in the right portion of FIG. 1, the gap becomessmaller from the center, i.e., the supply channel 3, toward the outside,such that a nozzle effect is produced.

[0046] The fluid is transported to the filter membrane 2 with a velocitycomponent that extends perpendicular to the surface of the filtermembrane in the cross-sectional region of the supply channel 3. Asubstantial portion of the flow is then conveyed parallel to the surfaceof the filter membrane 2 through the gap 5 that serves as the crosschannel, namely as indicated by the arrows V_(q) (for crossflowvelocity). The cross channel 5 then transitions into a return channel 6that conveys the fluid emerging from the gap 5 back into the supplychannel 3. The end of the return channel 6 that leads into the supplychannel 3 is realized in the form of a pipe end 6′ that is bent in thedirection of the filter membrane 2 such that the returned fluid is fedback into the supply channel parallel to the main flow (see arrowF_(in)). In the embodiment according to FIG. 1, the supply channel 3contains a restriction 3′, wherein the narrowest point preferably liesin the vicinity of the outlet of the pipe ends 6′ of the return channel6. As described above, this restriction results in a higher flowvelocity and consequently a pressure drop in this region, such that thepressure on the outlet end of the return channel is lower than at thesurface at the filter membrane and at the inlet of the return channel 6.The described effect is additionally promoted by the cross-sectionalrestriction caused by the pipe ends 6′ that protrude into the supplychannel 3.

[0047] Inserts that also serve to reduce the flow cross section may bearranged in the vicinity of the outlet opening of the return channel 6alternatively or additionally to the cross-sectional restriction 3′,said inserts having the shape of, for example, spherical elements 9′.

[0048] A pump 6″ may be alternatively or additionally arranged in thereturn channel 6 in order to realize the return flow of the fluid.

[0049] Several tubular, axially symmetrical baffle elements 7 and 8, aswell as a spherical baffle element 9, are arranged in the supply channel3. The entire device 1 including the main body, the baffle elements,etc., is realized as axially symmetrical relative to the axis A. In theembodiment shown in FIG. 1, the baffle element 7 has a purelycylindrical shape and the baffle element 8 has the approximate shape ofa funnel with perforations 10 that ensure a more intense turbulence.These perforations may also be arranged to axially symmetrical relativeto the axis A.

[0050] Relative to a Cartesian coordinate system with the two axes x andy that is illustrated in the lower right corner of FIG. 1, the fluidflows along the baffle elements 7, 9 in the direction of the arrowF_(in), namely in the direction of the negative y-axis parallel to theaxis of symmetry A. The fluid then flows in the direction of the inflowside of the filter membrane 2 through the perforated baffle element 8.This causes a turbulent flow to be produced on the inflow side of themembrane 2, wherein this turbulent flow is produced particularly, butnot exclusively in the center of the membrane inflow side. Thisturbulent flow at least decelerates the depositing of materials retainedby the membrane 2 on the inflow side. In addition, a flow with a highcrossflow velocity (V_(q)) is produced parallel to the membrane 2 in thegap 5, wherein this flow transports the retained materials along theinflow side of the membrane (parallel to the x-direction). The crossflowin the gap 5 is preferably turbulent such that materials situated on themembrane 2 are “swirled up” and transported away with the crossflow.

[0051] The retained materials are removed or separated from thecrossflow, and the crossflow that is subsequently essentially free ofretained materials is conveyed back into the main flow, i.e., the supplychannel 3, via the return channel 6. This makes it possible to clean themembrane 2 without interrupting the filtration process.

[0052] Different variations of the return channel design are illustratedin the right and the left half of FIG. 1. In the left portion, thereturn channel extends obliquely upward in a straight fashion. In theright portion, the return channel initially extends vertically upwardand then back into the return channel 3 in the form of an angled-offsection, a horizontal section and a downwardly bent section. Naturally,the return channel can also have a different shape.

[0053] If the distance from the center of the membrane 2 to its edge isnot excessively long, the pressure drop along this distance isrelatively small, and a sufficient transport effect is provided, as isdescribed in approximation by the Law of Hagen-Poiseuille${\Delta \quad p} = {{\frac{V}{t} \cdot R} = {\frac{{\delta\eta}\quad L}{\pi \quad r^{4}}\frac{V}{t}}}$

[0054] where L=length of a pipe and r=radius of the pipe, whichapproximately corresponds to B.

[0055] The flow state of a fluid is described with the aid of theReynolds number Re that is defined by the following equation (1):

Re=2rρv/η  (1)

[0056] In this case, r (m) is the radius of the cross-sectional areathrough which the fluid flows, ρ is the density of the fluid (whichdepends on the fluid temperature), v (m/s) is the flow velocity and η(Pa•s) is the viscosity of the fluid. For example, the correspondingvalues for water are: η=1.8×10⁻³ Pa•s (at 0° C.) and 1.0×10⁻³ Pa•s at20° C.; ρ=1000 kg/m³. The flow is laminar for Re values<approximately2000 and turbulent for Re values>approximately 3000 (the flow state isunstable in the range 2000<Re<3000 and changes from one type to theother).

[0057] According to equation (1), the fluid flow can be transformed intoa turbulent and/or laminar flow by choosing the parameters r, v (and,under certain circumstances, also the temperature T and consequently theviscosity of the fluid) accordingly. The present invention utilizes thiscapability. If the corresponding parameters change over a channellength, the flow state also changes accordingly.

[0058] The example of the filter device illustrated in FIG. 1 has, forexample, the following parameter values (in SI units): ρ=1000 kg/m³,η=1.8×10⁻³ Pa•s, r=0.5×10⁻³ m (0.5 mm)=B/2; v=10 m/s (pressuredifferential 1 bar). These values result in a Re value≈5500 for the gapregion 5 between the underside 12, 16 and the membrane 2, i.e., aturbulent flow with a very high flow velocity v.

[0059] If the parameters are varied (particularly r and B) over thefluid path, it is possible to produce zones with a turbulent flow andzones with a laminar flow, and the pressure drop along the upstream sideof the membrane 2 can be adjusted accordingly. Naturally, this alsoapplies to any other region of the fluid flow, in particular, the regionof the baffle elements 7, 8 and 9. A turbulent flow with an intensecrossflow component is produced in the region z of the surface of themembrane 2 due to suitable parameters and shape of the baffle elements.

[0060] A second embodiment of the filter device according to the presentinvention is shown in the right portion of FIG. 1. This embodiment isrealized in similar fashion to that shown in the left portion of FIG. 1.However, the underside 12 of the main body 4 does not extend parallel tothe membrane 2 as is the case with the underside 11 [sic; 16] in theleft portion of FIG. 1, but rather forms a gap 5 that narrows in thedirection of the edge of the membrane 2. The contour can, in particular,be defined in accordance with the Hagen-Poiseuille equation such thatthe respective ratios dV/dt×L/r⁴ and dV/dt×L/B⁴have a constant value.According to the Hagen-Poiseuille equation, this leads to a constantpressure drop (Δp) over the length L on the inflow side of the membrane2. Depending on the chosen parameters (e.g., dV/dt, L, r or B, and η),the pressure drop required to transport away retained materials can bedefined in the desired fashion. This improves the transport efficiencyand makes it possible to reduce the pressure on the inflow side of themembrane 2 (reduced energy requirement, smaller pumps, and lower pumpoutputs). In this embodiment of the filter device 1, no additionalexpenditure of energy and no additional media (e.g., gases or the like)are required for cleaning the membrane 2 because the membrane is cleanedautomatically.

[0061] According to one additional development of the invention, atleast one electrode 14 can be additionally provided. In the exampleshown in the left portion of FIG. 1, a primary electrode 14 is arrangedin the spherical baffle and two secondary electrodes 15 are arranged inthe main body 4. Naturally, the number and arrangement of the electrodesmay also be chosen differently. For example, it would possible toarrange electrodes 11, 17 on the main body 4, for example, directly onits respective undersides 16 and 12. The electrodes may also be arrangedsuch that they fulfill the function of baffles in the gap 5. In thiscase, the electrodes should be encased in order to provide a protectiveshield against the fluid. The electrodes make it possible to generateelectric fields in the fluid and, in particular, in the vicinity of thefilter membrane 2, these electric fields preferably being pulsed suchthat the separation of retained materials from the membrane 2 ispromoted. Another electrode 19 may be provided on the downstream orupstream side of the membrane 2, wherein this additional electrodenaturally has a wider mesh than the membrane, and has a repellent effecton the solid particles.

[0062] The electrodes partially protrude into the fluid flow, inparticular, into the gap 5, as well as in the direction of the membrane2 from the spherical baffle element 9, wherein the electrodes are, forexample, wired as an anode. Electrical conductors that are wired as acathode are applied to the inflow side of the membrane. If a currentpulse is transmitted through the fluid via the cathode and the anode,small gas bubbles are produced, e.g., due to the electrolytic breakdownof water, such that a layer of deposits which may have formed on theinflow side of the membrane is broken apart and its separation from themembrane is at least facilitated. This also causes extremely reactivefree radicals to be produced which at least impair or even entirelyprevent the formation of biofilms.

[0063] The values of the current pulses lie, for example, between a few10 mA/cm² and a few 100 mA/cm². The number of current pulses per hourmay lie between 2-12 or more, depending on possible fouling, thecomposition of the unfiltered fluid, and the membrane values. Theduration of the current pulses lies between a few seconds (1-5 sec) anda few tens of seconds.

[0064] The number of electrodes assigned to the membrane surface (numberof electrodes/membrane surface) naturally can be chosen in accordancewith the respective requirements. In one embodiment, eight electrodesare provided in the main body and one electrode is provided in thespherical baffle element. The electrodes can also be protected from thefluid by means of a casing that does not influence the electric field.This additional feature can impair or entirely prevent the formation ofa fouling layer, and makes it possible to significantly increase thethroughput of the fluid through the membrane 2. Consequently, the samethroughput can be achieved if the filter surface or the pressure (pumpoutput, energy consumption) or the fluid velocity is reduced. Theeffects on the costs of the filtration process are quite apparent.

[0065]FIG. 1 also shows that a separating device 20 is arranged at theradial end of the gap 5 and in the region of the transition to thereturn channel 2 [sic; 6]. This separating device separates thematerials being transported to the edge of the membrane such that theycannot be reintroduced into the return channel. This separating device20 may simply consist of an annular container that may be realized asaxially symmetrical relative to the axis A and receives the crossflow ofthe fluid. If the retained solid particles are heavier than the fluid,they precipitate and are “collected” in the separating device 20. Thesolid particles are intermittently removed from the separating devicewithout having to interrupt the filtration process.

[0066]FIG. 2A shows an embodiment of a separating device 20 that ismounted on the main body 4. This separating device consists of acollection container that is provided with a conventional inexpensivefilter element 22 (e.g., of cellulose, metal or the like) on the sidethat is connected to the main body 4, said filter element preferablyhaving a larger nominal pore diameter than the membrane 2. Theseparating device 20 is, for example, tightly connected to a projectionor flange 24 of the main body 4 by means of a simple locking mechanism23. A valve 24 that, for example, may consist of the simple rotary slidevalve of conventional design can be provided between the lockingmechanism 23 and the edge of the membrane 2. The valve 24 is brieflyclosed during the filtration process in order to allow the installationor replacement of the separating device 20 without having to interruptthe filtration process.

[0067] The function of the separating device 20 is described below.During the filtration process, the materials retained by the membrane 2are transported to the membrane edge along the upstream side of themembrane 2, i.e., in the direction of the separating device 20. Due tothe larger pore width of the conventional filter element 22, theretained materials can be transported into the collection container 20through the filter element 22. The “dead zone effect” of the collectioncontainer 20 lowers the probability of the materials accumulating in thecollection container being reintroduced into the fluid flow through thefilter element 22. The entire space (collection container 20, gap 5 andreturn channel 6″) is filled with fluid. Due to this measure, theseparating device 20 effects a filtration wherein the separation of theretained materials is achieved by means of a suitable combination offlow progression (including the dead zone=collection container 20 andthe return channel 6), crossflow and filtration with the membrane 2.

[0068] In order to promote the return of the fluid into the main flowF_(in), the return channel 6 may, for example, be realized with avarying diameter, i.e., a diameter that is not constant. For example,the diameter of the return channel 6 may decrease from the valve 24 tothe point at which the return flow is reintroduced into the main flow.This not only increases the flow velocity and decreases the pressurealong the return channel, but also results in a certain pressuredifferential that “drives” the return flow in addition to the flowrestriction 3′ (or 9′ in FIG. 1).

[0069] The fluid that is at least partially or largely free of theretained materials is fed back into the main flow F_(in) via the returnchannel 6 formed in the main body 4 and conveyed back to the membrane 2with this main flow, such that the cleaning circuit is closed. Whenobserving one volume element in the fluid flow, materials are retainedby the membrane 2 in a first passage through the cleaning circuit,transported to the separating device 20 with the aid of the crossflow,and deposited therein to a certain degree. Subsequently, the fluid thatnow has a significantly lower content of retained materials isreintroduced into the main flow. If the residual content of materials tobe retained is still excessively high in the observed volume element,the filtration process at the membrane 2 is repeated. This results in adynamic equilibrium, in which the membrane 2 simultaneously andcontinuously carries out the desired filtration process, the membrane 2is automatically cleaned, and the retained materials are removed fromthe fluid flow.

[0070] The separating device 20 may be manufactured of stainless steel,plastic or any other customary materials used for such applications.

[0071] In other embodiments, the channels and/or the flow progressioncan be varied in accordance with the respective requirements. Forexample, the filter element 22 can be arranged at an angle other than90° relative to the crossflow (V_(q)) emerging from the gap 5 (see FIG.2B). It is also possible to arrange the inlet opening into thecollection container 20 at its upper end such that materials introducedinto the container settle out even better on its bottom, and are hardly,or preferably not at all, able to escape into the return channel 6.Other conceivable inflow geometries are schematically illustrated inFIG. 2C. The flow can be conveyed into the collection container 20 bymeans of an appropriate deflection 26 of the gap 5 toward the returnchannel 6.

[0072] If the inflow opening into the collection container 20 is madecomparatively small, it may, if so required, be possible to omit afilter element 22 entirely. The primary function of the filter element22 consists of preventing or at least impairing return of the materialsthat were previously retained by the membrane 2, and were introducedinto the collection container 20, from the collection container into thereturn flow and consequently into the return channel 6. The probabilityof particles being returned into the return flow and into the returnchannel 6 can also be reduced with constructive measures. One example ofthis is schematically illustrated in FIG. 2D, according to whichappropriate guide elements 27, a concavely curved inflow edge 28 that isinclined toward the gap 5, and another guide plate 29 that forms anarrow gap 30 together with the inflow edge 28, are provided for thispurpose. This geometry of the opening can be integrated into theseparating device 20 such that standard elements can be used for theconnections and piping of the separating device 20. The baffle elements27 and 29 contained in the separating device 20 narrow the inlet channelinto the collection container 20 from the connection 23 to asubstantially smaller opening 30. The probability of materialsintroduced into the collection container 20 being returned through theopening 30 is approximately proportional to the ratio between thesurface of the opening 30 and the surface of the entire inner wall ofthe collection container 20. It should be mentioned that the entirespace (20, 5, 6) is filled with fluid once the device is “broken in,”and that separation is based on an “enrichment or concentration” of theretained materials in the collection container 20. The inventionutilizes the inertia of these materials during their transport into theseparating device 20.

[0073] The mounting mechanisms of the collection container 20 may alsobe modified. It would be conceivable to utilize threaded and/orclamp-type connections if the tightness of the connection can beensured.

[0074] It is preferred to utilize more than one separating device 20 ina filter device according to the present invention, namely at least twoor more separating devices. If four separating devices are provided,they are symmetrically arranged on four sides. The utilization of morethan one separating device also ensures that the separation process andconsequently filtration at the membrane 2, as well as its cleaning, cancontinue during maintenance procedures (e.g., replacement, etc.).

[0075] The functional principle of the separating device described inthis application represents an additional filtration that, if sorequired, can also be utilized in the form of an independent and/or solefiltration principal. If a suitable design is chosen (scaling, channelprogression, etc.), the inertia of masses of materials that arecontained by and should be removed from the fluid can be utilized forcollecting these materials in “dead zones” in a circulating fluid flow.In this case, an increased flow resistance needs to be provided in theoutflow channel for the depleted fluid at the location at which thecircuit begins (e.g., by means of membranes, baffle elements, a varyingchannel diameter, etc.), and wherein the separating device may, if sorequired, also be realized in the form of a centrifugal filter.

[0076]FIG. 3 shows that individual elements also have corrugated ornon-planar surfaces. In FIG. 3A, the baffle element 7 is realized in acorrugated or ribbed fashion while the main body 4 has a smooth wall.Naturally, the main body 4 may also be corrugated. In FIG. 3B, the mainbody has a corrugated underside 12. This makes it possible to producespecific zones with a turbulent flow in regions that are spaced apartfrom the respective adjacent element by greater distances. The membrane2 naturally may also be corrugated.

[0077]FIG. 4 shows how a baffle element in the form of a shell isinstalled in the main body 4. The main body 4 comprises at least twosections 31 of an internal thread which protrude into the supply channel3, with three sections being provided in the embodiment shown, whereinthe radially inward directed surfaces 32 form the inside thread. Anaxially symmetrical baffle element 33 is provided with a correspondingexternal thread and is screwed into the internal thread 32. Other baffleelements, e.g., the baffle element 30, are also provided withcorresponding threads. This shell-like structure can be arbitrarilyexpanded depending on the respective application.

[0078] Naturally, the baffle elements can also be installed withmounting devices other than threaded connections. For example, the twoparts (main body and baffle element ) may be respectively provided withpins and correspondingly designed grooves such that the part providedwith the pins can be simply inserted into the grooves of the other partand interlocked therein. The two aforementioned mechanisms forinstalling the baffle elements provide a high degree of flexibility withrespect to maintenance and parts to be replaced. Depending on theprocess conditions, the fluid used, etc., the baffle elements andnaturally—if so required—also the main body can be exchanged with othercomponents. The spherical baffle element 30 is interlocked, for example,with the baffle element 33 with the aid of projections.

[0079] In another embodiment of the invention that is illustrated inFIG. 5, the spherical baffle element 9 is supported on a spring 35 thatis mounted on the baffle element 8. The spring constant of the spring 35also makes it possible to realize a flow control. If a pressure surgeoccurs in the inflowing fluid, the spherical baffle element 9 isdisplaced in the direction of the membrane 2 against the force of thespring 35 such that the flow channel between the spherical baffleelement 9 and the baffle element 8 becomes narrower. An essentiallyuniform fluid supply to the membrane 2 can be realized in this fashion.In addition, the membrane is protected from pressure surges that coulddamage the membrane and could also lead to an interruption of thefiltration process.

[0080] The dimensions of the individual components of theabove-described filter device can be chosen arbitrarily in order toadapt the filter device to the respective application. The principle ofthe invention certainly can also be utilized on an industrial scale,e.g., in wastewater treatment.

[0081] The filter device with the main body and the baffle elements canalso be installed in a housing of stainless steel or other materials.This housing can be inserted into conventional pipelines by means ofstandard connections.

1. Filter device for filtering fluids, with a filter membrane and adevice for producing a crossflow directed tangentially relative to thesurface of the filter membrane, and with a main body, arranged upstreamof the filter membrane, that has a supply channel extendingperpendicular to the surface of the filter membrane, a gap betweenitself and the filter membrane in order to produce the tangentiallydirected crossflow, and a return channel, characterized by the fact thatthe return channel (6) is fluidically connected to the supply channel(3), and by the fact that a device (3′,6′,6″) is provided for realizingthe return flow of the fluid through the return channel (6).
 2. Filterdevice according to claim 1, characterized by the fact that the devicefor realizing the return flow of the fluid consists of a flowrestriction (3′) arranged upstream of the filter membrane (2).
 3. Filterdevice according to claim 1 or 2, characterized by the fact that thedevice for realizing the return flow contains at least one tube end (6′)that protrudes into the supply channel (3) and the outlet of whichintroduces the returned fluid into the supply channel (3) parallel tothe longitudinal axis of the supply channel (3).
 4. Filter deviceaccording to one of claims 1-3, characterized by the fact that thedevice for realizing the return flow of the fluid comprises a pump (6″)that is arranged midway in the return channel (6).
 5. Filter deviceaccording to one of claims 1-4, characterized by the fact that a solidsseparator (20) is arranged in the return channel (6).
 6. Filter deviceaccording to one of claims 1-5, characterized by the fact that at leastone baffle element (7,8,9) is arranged upstream of the filter membrane(2) in order to produce a turbulent flow.
 7. Filter device according toclaim 6, characterized by the fact that at least one of the baffleelements (8) contains one or more holes (10).
 8. Filter device accordingto claim 4 or 7, characterized by the fact that the baffle elements(7,8,9) are inserted into and connected to one another, preferably bymeans of threaded sections or pins.
 9. Filter device according to one ofclaims 1-8, characterized by the fact that an electrode (15) forgenerating an electric field in the fluid is arranged at least in themain body (4).
 10. Filter device according to claim 9, characterized bythe fact that at least one of the baffle elements contains an electrode(14).
 11. Filter device according to claim 9 or 10, characterized by thefact that the electrodes generate an alternating electric field. 12.Filter device according to one of claims 1-11, characterized by the factthat the gap (5) between the underside (12) of the main body (4) and thefilter membrane (2) has a width (B) that increases [sic] radiallyoutward.
 13. Filter device according to one of claims 3-12,characterized by the fact that at least one baffle element (9) ismovably supported, preferably by means of a spring (35).
 14. Filterdevice according to one of claims 2-13, characterized by the fact thatan additional filter element (22), as well as a valve arrangement forshutting off the fluid supply to the solids separator, is arrangedbetween the gap (5) and the solids separator (20).
 15. Filter deviceaccording to claim 14, characterized by the fact that the additionalfilter element (22) is arranged relative to the inflowing fluid at anangle other than 90°.
 16. Filter device according to one of claims 5-15,characterized by the fact that the area of the opening (30) of thesolids separator (20) is significantly smaller than the area of theinner wall of the solids separator.
 17. Separating device according toone of claims 1-16, characterized by the fact that the return channel(6) has a varying diameter that preferably becomes smaller in the returnflow direction.